1. Field of the Invention
The present invention relates to selected curable and cured thermoset polymer thick film (PTF) compositions and their use, when cured, as electronic circuitry and the like.
More particularly, this invention relates to selected single-component curable PTF compositions that have a relatively long shelf life at room temperature (i.e., more than six months) and are curable in an infrared oven at relatively high temperatures and short times (e.g., at about 180.degree. C.-250.degree. C. for less than about 10 minutes) on a wide variety of substrates (e.g., phenolic paper-type printed wiring boards). Still further, this invention relates to selected cured PTF compositions which exhibited good flexibility, have good abrasion and solvent resistance, have acceptably good adhesion to a substrate as well as are stable at high temperatures (e.g., about 300.degree. C.) for short periods of time. And further, the present invention relates to a process of curing such PTF compositions.
2. Brief Description of the Prior Art
Thermoset polymer thick films have been used as screen-printed electronic circuitry on a substrate material for various electronic applications such as jumpers on printed wiring boards, key pads for computers and typewriters, and the like. These prior art polymer thick film compositions preferred to use thermoset polymers in them rather than thermoplastic polymers because the former are more solvent resistant as well as high temperature stable. The latter property is particularly needed when the PTF-coated substrate is subjected to a high temperature solder bath (i.e., about 300.degree. C. for up to 30 seconds) after the PTF application step.
Prior art thermoset PTF compositions have generally consisted of epoxy resin or mixture of resins, a curative (e.g., DICY), and a catalyst (e.g., urea). Presently, the commercially available thermoset PTF compositions exhibit either a long shelf life at room temperature (i.e., over six months) and require a high temperature cure (i.e., about 160.degree. C. or higher for 30-60 minutes) or exhibit a short shelf life (i.e., about two to three weeks) and a lower temperature cure (i.e., about 125.degree.-140.degree. C. for 30-60 minutes) in a convection oven.
Ideally, a thermoset PTF composition should possess the following combination of properties:
(1) exhibit a relatively long shelf life at room temperature (i.e., at least six months); PA0 (2) require a low curing temperature and short curing times in a convection oven (i.e., no higher than 125.degree. C. to 140.degree. C. for no longer than 60 minutes); as well as be curable in an infrared oven at higher temperatures (i.e., 180.degree. C. to 250.degree. C.) for very short times (e.g., less than 10 minutes). PA0 (3) be sufficiently conductive (or have low electrical resistivity) to carry an electric current; PA0 (4) be sufficiently flexible when cured to be applied to flexible substrate (or substrates which might flex during their operation); PA0 (5) have good abrasion resistance when cured so that they do not easily get rubbed or scratched off the substrate; PA0 (6) have acceptably good adhesion to the substrate when cured; PA0 (7) be stable at relatively high temperatures (at about 300.degree. C.) for short period of time such as those employed in solder bath treatments; PA0 (8) be easily prepared; and PA0 (9) be inexpensively applied to a substrate and cured without needing a complicated procedure. PA0 (1) providing a polymer thick film composition as defined above; PA0 (2) applying said composition to a substrate; and PA0 (3) curing said composition on said substrate.
One recent alternative class of thermoset PTF composition is disclosed in U.S. Pat. No. 07/403,235, filed on Sep. 5, 1989, now U.S. Pat. No. 5,049,313, patented Sep. 17, 1991, with Richard L. Frentzel as the named inventor. The U.S. patent application is incorporated herein by reference in its entirety. This class of thermoset PTF compositions comprises certain parts by weight of at least one phenoxy resin, at least one blocked isocyanate resin, a particulated electrically conductive material (i.e., silver flake or a mixture of graphite and carbon black), and, optionally, a nonhydrocarbon polar solvent. While members of this particular class of thermoset PTF compositions possess almost all of the above-listed combination of desired properties, those PTF compositions suffer the disadvantage of being unable to be cured on a phenolic paper-type printed wiring board substrates in an infrared oven because the high curing temperature of 235.degree. C. distorted the phenolic paper-type boards.
The PTF compositions of the present invention, unlike those disclosed in the prior patent application, are believed to have all of the desired combination of properties without that disadvantage.